The present invention relates to a method for analyzing the characteristics of a surface using ellipsometry, and more particularly a method for estimating the roughness and the contamination of a surface, e.g. a gold surface.
In the manufacturing of electronic card once the chip (or the chips in the case of Multi Chip Modules) is mounted on the substrate (e.g. organic or ceramic) it must be electrically connected to the circuits on the substrate (e.g. a printed circuit). The connection is usually done by means of very thin gold wires which are connected to gold pads on the substrate. This operation is called wire bonding and it usually consists of applying a reciprocal pressure and an ultrasonic vibration to the gold wire and the gold pad surface. Due to this operation the two gold surfaces penetrate one another with a xe2x80x9cdiffusionxe2x80x9d of some of the surface layer atoms which move from one surface to the other. The above described wire bonding may be adversely effected by the condition of the two gold surfaces, which should ideally be as flat and clean as possible. Any roughness or contamination of the surfaces should be avoided, otherwise these may result in a poor bonding strength.
The roughness of the surfaces reduces the actual contact area, while an unwanted contamination (e.g. carbon) of the gold surfaces could act as a barrier to the movement of the gold atoms. The surface roughness xcfx86 (i.e. the surface irregularity) could be mathematically expressed as the ratio between the volume occupied by the metal on the surface with respect to the volume occupied by air:
xcfx86=Vmetal/(Vmetal+Vair)xe2x80x83xe2x80x83(1)
The contamination may be caused by environmental pollution or by chemical deposited on the pads during the substrate manufacture.
The influence of surface roughness and contamination on metal/metal wire bonding is well known in literature: see as an example J. A. DiGirolamo, xe2x80x9cSurface roughness sensitivity of aluminium wire bonding of chip on board applicationsxe2x80x9d, ITL, 1989; or A. Schneuwly et al., xe2x80x9cInfluence of surface contamination on metal/metal bond contact qualityxe2x80x9d, Journal of Electronic Materials, 27(8), p. 990, 1997; or J. Krzanowski et al., xe2x80x9cThe effects of thin film structure and properties on gold ball bondingxe2x80x9d, Journal of Electronic Materials, 27(11), p. 1211, 1998; or M. Souma et al., xe2x80x9cBonding quality evaluation technology for semiconductor packagesxe2x80x9d, New Tech. Rep., 60 September, p. 58, 1998.
Given the problems that a bad wire bonding can cause to the electronic card, it is often required that the metal (e.g. gold) surface characteristics are checked before the bonding operations.
Many analytical techniques are known in the art, which can be used to inspect the metal (e.g. gold) surfaces in order to measure carbon contamination thickness and roughness before they undergo the wire bonding process.
The most common method for estimating the surface roughness is by analyzing the surface with highly precise instruments like an Atomic Force Microscope (AFM) or an Interferometer. The surface contamination (particularly the presence of carbon particles) can be detected with X-ray Photoelectron Spectroscopy (XPS), Auger Spectroscopy (AUGER), Secondary Ion Mass Spectroscopy (SIMS) or Ellipsometry.
Instruments like AFM and Interferometer inspect the surface morphology by measuring the distance from the probe to peaks and valleys.
The waves of the light incident on the surface change their amplitude according to the distance from the light source to the surface height: an Interferometer is capable of correlating this change with the surface height.
The interaction between the surface atoms and the instrument probe, that can operate in contact or no contact mode, generates the so called Van Der Walls forces that cause a deflection of the probe; the deflection, being proportional to the atom-probe distance, makes it possible to obtain the surface morphology with the aid of an AFM.
In the above cases the calculation of the percentage of the volume occupied by the metal (e.g. gold) on the analyzed surface is performed according to the following equation:
xcfx86=Vmetal/Vtotalxe2x80x83xe2x80x83(2)
where:
Vmetal=the metal volume calculated as the summation of every volume unit given by: ((the minimum value of all the valleys)+(the Mean Height of every Unit Area ))xc3x97(every correlative Unit Area)
Vtotal=(Surface Area)xc3x97((Mean height)+(RMS roughness)).
Of course Vtotal=Vmetal+Vair.
Mean Height is the average value of all the distances measured from peaks to valleys on the surface RMS=The square root of the average of the squares of the differences between Mean Height and the height of every peak. Surface Area is the real size of the region analyzed and it is calculated by the addition of all the Unit Areas. Unit Area is the double value of the surface corresponding to the triangle connecting the three nearest neighbour data points. The surface or xe2x80x98realxe2x80x99 area depends on the roughness and it is different of course from the xe2x80x98apparentxe2x80x99 area, that is the macroscopic dimension of the sample.
XPS, AUGER and SIMS are very sensitive analytical techniques, that use sophisticated instruments to detect very thin layers (down to one nanometer) of surface films. These instruments are equipped with a source of particles that irradiates the sample and a detector that analyzes the energy of the particles emitted from the sample. The energy of the emitted particles depends on the energy of the incident particle and the bonding energy of the particle in the surface atoms, because of the energy transfer from the incident particle to the atom present on the surface. It is possible by these techniques identifying which elements are present on the surface, their chemical bonding and atomic percentage. These instruments are also provided with a sputtering system that etches layer by layer, with a resolution of few Angstrom, the surface of the sample, and gives the depth profile concentration for every element. Each technique uses a particular source for bombarding the sample and a detector to measure the energy of the particles emitted from its surface:
XPS irradiates the sample with X-ray photons and detects the electrons;
AUGER source irradiates electrons and detects electrons;
SIMS bombards the sample with atoms or ions and analyzes the emitted ions (secondary).
All the above described methods of the prior art have the drawback to be very complex and slow. Furthermore the In instruments required are very expensive. it is known in the art to use ellipsometry to detect the surface contamination of metal, e.g. see R. M. A. Azzam and N. M. Bashara, xe2x80x9cEllipsometry and polarized lightxe2x80x9d, North-Holland, 1987; or K. Riedling, xe2x80x9cEllipsometry for industrial applicationxe2x80x9d, Springler-Verlag, 1988; or V. S. Brusic et al., xe2x80x9cMANCA TITOLOxe2x80x9d, J. Vac. Sci. Technol., A8(3), 2417, 1990.
Ellipsometry represents for some applications a good alternative to XPS, AUGER and SIMS. It is a cheap and fast technique, used widely for measuring the thickness of films present on metal or semiconductor surface. Ellipsometry is based on the fact that a monochromatic electromagnetic wave changes its intensity and state of polarization if it strikes non-perpendicularly the interface between two dielectric media, that is represented by a substrate coated with a film. The ellipsometer polarizes linearly the light beam before it strikes the sample surface; linearly (or circular polarization) means that the light gets only two perpendicular components having the same amplitude. The beam, after going through the interface between the surface film and substrate, is reflected and it changes its polarization, i.e. both the ratio amplitude of the two components "psgr" and their phase xcex94 are modified. The two components of the reflected light are no longer mutually perpendicular and have a different amplitude; this is the reason why the polarization becomes elliptical and the technique is called ellipsometry. The ellipsometer measures the experimental values of. the two components of the reflected light, giving the value "psgr" and xcex94. The first is calculated by multiplying the ratio of amplitudes of the incident beam by the amplitudes of the reflected beam; the value xcex94 comes from the difference of their phases.
The main purpose of ellipsometry inspection according to the prior art methods is to measure the thickness of oxide and organics films present on low absorbing light substrates. The roughness is considered a xe2x80x98disturbxe2x80x99 for ellipsometry and many mathematical models have been developed to correct the measurement errors induced by roughness.
The above described ellipsometry inspection of the prior art does not give useful indication of the roughness of the analyzed surface. In general none of the above techniques is capable to measure contemporarily roughness and contamination.
It is an object of the present invention to alleviate the above drawbacks of the prior art.
According to the present invention, we provide a method for analyzing the characteristics of a surface coated by a contaminant film, the method comprising the steps of:
calculating, for a plurality of predetermined values of the surface roughness xcfx86 and for a plurality of predetermined thickness T of contaminant film, the expected values of the ratio "psgr" between the amplitudes on the two polarization planes of a beam bi incident on said surface multiplied by the ratio of the amplitudes of the reflected beam br on the two polarization planes, and the value of the difference xcex94 between the phases on the two polarization planes of the incident beam bi and the reflected beam br;
measuring the value of "psgr" and xcex94 of a surface;
determining the value of xcfx86 and T for that surface by comparison with the plurality of expected values.
Various embodiments of the invention will now be described in detail by way of examples.